Horizontal Resolution A Key Index

August 1, 1986|By HANS FANTEL, New York Times News Service

The brief midsummer lull in the electronics trade provides a welcome chance to reconsider some of the fundamental concepts of video that are often glossed over in the headlong rush of developments. Such concepts should be re-examined periodically, for it is only in terms of these basic principles that we can evaluate the ceaseless stream of innovation.

Among all the theoretical yardsticks of video performance, none is invoked more often than horizontal resolution. This single measurement is widely regarded as a general index of quality for a VCR, a TV set or a video camera.

The term resolution, in this context, has nothing to do with steadfastness of purpose or strength of will. Rather, it signifies the amount of detail visible in an image. In photography, for example, a fine-grained film is said to have high resolution because it can ``resolve`` (i.e., make recognizable) smaller details within the picture than a coarse-grained film.

In video, resolution primarily involves two factors. One is the number of scanning lines per frame. These lines go across the screen, just as a line written by a typewriter goes across the paper. Their number is fixed by federal regulation at 525 lines -- not a very high quantity by present technical capabilities. But unless all our video equipment is to be declared obsolete and thrown on the scrapheap, we are stuck with it.

The other factor determining the resolution or sharpness of the picture is the number of separate points that can be illuminated along each scanning line. This is what is known as horizontal resolution; it varies from set to set and is also stated as a number of lines.

The specifications for a VCR or for a TV set usually state its horizontal resolution as a number of lines rather than a number of points. The reason is that the actual count is made by means of a test pattern projected on the screen in which lines rather than points are counted horizontally across the picture.

Of course, no detail can be displayed unless it is contained in the signal itself, and the amount of detail the signal can accommodate depends on its bandwidth -- i.e., the frequency range available for the electronic transmission of the picture. A good video broadcast signal covers a bandwidth of roughly 4 megahertz, which means there`s a difference of 4 million cycles per second between the highest and lowest frequencies of the signal.

A few of the best TV sets on the market today can reproduce nearly all of the transmitted signal and thereby achieve a horizontal resolution upward of 300 and possibly close to 400 lines. By contrast, the average set fails to cover the full broadcast bandwidth and typically attains only about 240 to 280 lines of horizontal resolution. These figures are clearly stated in the specifications provided by many manufacturers and give a reliable clue to image sharpness.

The signal obtained from a VCR is inherently inferior to that obtained from a direct broadcast (received with a proper antenna under good conditions) or the signal conveyed by a high-quality cable system. Even when operating at the optimal tape speed, VCRs rarely reach a horizontal resolution exceeding 250 to 280 lines, which is indicated on some but not all specification sheets.

Unfortunately, greater bandwidth and higher horizontal resolution can be mixed blessings. Along with yielding more detail, they also make the display more sensitive to ``video noise`` in the form of grainy image textures. Many TV sets and some VCRs therefore have a so-called sharpness control by which viewers may vary the bandwidth and hence the horizontal resolution. Though of primary importance, horizontal resolution is only one of several factors that determine picture quality. Many of the others, such as color purity, cannot be numerically expressed and must be evaluated subjectively by watching test patterns on the screen. In contrast, horizontal resolution represents a clearly quantifiable aspect of video performance, which accounts for its wide acceptance as a general figure of merit.